No, I don't misunderstand at all. Part one, which I suspect no longer gets visited very often, has no need to stay near the top. Part two, will stay near the top naturally because it's popular. As it is, I can't tell at glance whether there's anything new in the thread because it's always directly at the top. (This is partly because this particular forum software doesn't mark single threads as read or not.) As a result, I probably miss things that I might have enjoyed otherwise.
(I may be more sensitive to this than others because when I was laid up last winter on my back last winter, scrolling was physically painful. I bothered me every time I had to scroll year's old threads to get to new content.)
Another forum to which I belong has an "Archives" sub forum. It's meant for things like part one of "today" thread. Easy to find, but you don't have current threads cluttered to with cruft. (The current half of the thread belongs here, though -- but doesn't need to be pinned.)